The present invention relates to an optical system for an optical pick-up which is capable of recording data to and/or reproducing data from one or more kinds of optical discs having different thicknesses of cover layers.
There are various types of optical discs such as a CD (compact disc) and a DVD (digital versatile disc). The DVD has the cover layer thinner than that of the CD, and has a data recording density larger than that of the CD. For supporting both of the CD and DVD, the optical pick-up is required to have a configuration which enables to correct a spherical aberration which changes depending on the thickness of cover layer of an optical disc being used and to change NA (numerical aperture) of a light beam so as to attain a diameter of a beam spot suitable for the data recording density of the optical disc being used.
In general, the optical pick-up has a light source for emitting the light beam, a coupling lens, and an objective lens. The coupling lens has the function of collimating the light beam emitted by the light source, or changing the degree of divergence of the light beam. That is, the coupling lens is employed in the optical pick-up to suppress aberrations or to enhance the efficiency of use of light.
There is a demand for decreasing the number of optical components in the optical pick-up to decrease the cost of the optical pick-up and to further downsize the optical pick-up. Each of Japanese Patent Provisional Publications No. HEI 8-62496, HEI 8-334686, SHO 64-25113 and HEI 2-223906 discloses an optical pick-up which does not require the coupling lens. The optical pick-up disclosed in each of the publications is configured to form a relatively large beam spot suitable for the optical disc having a relatively lower recoding density and having a relatively thick cover layer such as the CD or CD-R. In the optical pick-up, the relatively large beam spot is attained by a relatively small NA.
More specifically, the optical pick-up disclosed in each of the publications is designed to suppress aberrations specifically for the CD or CD-R. In this optical pick-up, an on-axis aberration (spherical aberration) and/or aberrations caused by odd-axis light (including principally a coma and astigmatism), which occur when the objective lens is shifted along an optical axis of the objective lens for a focusing operation or when the objective lens is shifted in a direction perpendicular to the optical axis for a tracking operation and etc., are suppressed specifically for the CD and CD-R.
However, when the optical pick-up disclosed in each of the publications is used to record data to and/or reproduce data from the DVD, the effect of the amounts of aberrations caused when the objective lens shifts due to the focusing operation or the tracking operation increases to a degree that the recording or reproducing operation is affected.
The reason is that the DVD has the relatively high data recording density, and therefore it is necessary to form a relatively small beam spot by using an objective lens having a relatively high NA. Also, the tolerance to the aberrations of the DVD is smaller than that of the CD. For this reason, the optical pick-up is required to have the coupling lens to support the optical disc having relatively high recording density.
CD/DVD compatible optical pick-up devices configured to record data to and/or reproduce data from both of the CD and DVD have also been developed and widely used. In such a CD/DVD compatible optical pick-up, the same problem may occur when the configuration of the optical pick-up disclosed in one of the above mentioned publications is employed in the CD/DVD compatible optical pick-up.
That is, although the CD/DVD compatible optical pick-up has to suppress aberrations for each of the plurality of types of optical discs, such a requirement can not be accomplished when the configuration of the optical pick-up disclosed in one of the above mentioned publications is employed. For this reason, the CD/DVD compatible optical pick-up is also required to have the coupling lens.